Chromium-containing diffusion coatings for metals



Unite States Patent 3,377,196 CHRQMIUM-CONTAINING DIFFUSION COA'IINGS FOR METALS George V. Sneesby, Canoga Park, Calif, assignor to North American Rockwell Corporation, a corporation of Delaware No Drawing. Filed Sept. 22, 1965, Ser. No. 489,374 7 Claims. (Cl. 117-114) ABSTRACT OF THE DISCLOSURE This invention relates to a method of providing a chromium-diffusion coating on a base metal or alloy, and more particularly to a method for providing a chromium dilfusion coating on a ferrous metal.

Industrial and commercial uses of metals at high temperatures and in corrosive environments have continually increased, and the number of operations requiring metals with improved special properties is steadily rising. For a great many uses, it is essential that metal parts resist oxidation and other chemical surface reactions at high temperatures and abnormal conditions. Further, it is frequently desirable to have metal alloys of variable composition, for example, a very hard, corrosion-resistant surface, and a base having good working characteristics properties which frequently are not found with an alloy of uniform composition. Metals having corrosion-resistant surfaces at high temperatures are required, for example, for the following items: furnace parts, turbine blades, petroleum refinery equipment, jet engine sheet metal work, superheater tubes, and chemical plant equipment; the list of applications for ferrous metals with corrosion-resistant surfaces is virtually limitless. Thus, a process which can efficiently improve the temperature corrosion properties of metals, particularly ferrous metals, is of considerable interest.

Diffusion coatings on metals and alloys have been used to change one Or more metallurgical properties, for example, to produce an atmospheric oxidationaesistant surface, a surface resistant to the action of specific chemicals, and a wearand oxidation-resistant surface. It may be convenient to first make a particular piece from an alloy which is in easily formable condition, and then add the diffusion layer coating to obtain the desired property. A diffusion layer, which usually involves formation of an intermetallic compound or a solid solution of the coating metal with the base metal, has distinct advantages over a simple plating layer.

Chromium is widely used to improve the surface properties of ferrous metals, and also of refractory metals, such as molybdenum. A diffusion coating of chromium is generally applied by so-called pack cementation methods wherein a base metal part is placed in a high temperature retort containing a powdered chromium composition, an inert filler, and a halide salt. Upon heat-' mg, chromium halide vapors form which deposit upon the part and result in the formation of a chromium diffusion coating. Among the drawbacks of such processes are difficulties in coating complex shapes, particularly objects with hidden recesses or bores. The parts must be cleaned'prior to use, and time is consumed in packing the part and the chemicals in the retort, and then later in unloading the assembly. These operations increase the cost of such processes. Further, pack methods have been generally restricted to rather small parts and are not utilizable for large, finished articles because of equipment limitations. The pack methods are alsonot adapted to the formation of chromium-containing alloys of very high corrosion resistance, for example chromium-nickel, which on a ferrous metal base would form an Fe-Cr-Ni alloy, equivalent to a stainless steel or superalloy cladding on a low alloy or carbon steel base.

In US. Patent No. 3,186,865, assigned to the assignee of this invention, there is disclosed a method of forming a chromium-containing diffusion coating on a dissimilar base metal by placing the base metal in a molten lithium bath containing chromium, and optionally nickel, dissolved therein and maintaining the base metal in the bath until a chromium or chromiumnickel diffusion layer of desired thickness is obtained.

While the process described in US. Patent No. 3,186,- 865 leads to high quality chromium diffusion coatings, and particularly chromium-nickel diffusion coatings, further significant economic and other improvements may be obtained by the present invention which eliminates certain disadvantageous features present in the patented process.

Accordingly, an object of the present invention is to provide an improved method of forming a chromiumcontaining diffusion coating on base metals and alloys.

Another object is to provide a method of applying chromium upon a ferrous metal base to improve the oxidation and corrosion resistance of such base metals.

Another object is to provide a method wherein the chromium-containing diffusion material may be applied to all surfaces of a base metal independent of the configuration or shape of the base metal.

Still a further object is to provide a chromium-containing surface diffusion coating of a graded alloy of variable composition on a ferrous metal in order to increase its resistance to high-temperature surface corrosion.

In accordance with the present invention, a chromiumalloyable base metal is provided with a chromium-containing diffusion coating by placing the base metal in a molten calcium-sodium bath containing chromium values dissolved therein, and then maintaining the base metal in the bath for a sufficient time for the dissolved chromium to diffuse into the base metal to form a diffusion coating containing chromium on the base metal.

Because of the need and commercial importance of protecting ferrous base compositions, and because of the commercial acceptability of chromium coatings in this regard, a particularly preferred and commercially significant aspect of this invention is the providing of a chromium diffusion coating on a ferrous base metal. However, for certain other specialized applications, material such as cobalt or nickel may be codeposited with the chromium, and other base metals may be used, such as the refractory metals and those selected from the groups IV-B and V-B, and the first triad of the Periodic Table.

It is an essential feature of this invention that a molten calcium-sodium bath be used from which the chromiumcontaining diffusion coating is deposited. Somewhat surof Group VIII prisingly, since sodium itself it not operable. as a transfer I agent for forming a chromium diffusion coating, when sodium is combined with molten calcium in up to 50v percent by weight of the calcium-sodium content, distinct advantages are obtained which make the use of this bath commercially superior to that of calcium alone. Generally amounts of sodium of less than 5 weight percent other donor.

are of lesser significance. Thus, the use of sodium serves to lower the operating temperature, which is an important consideration in the practice of diffusion coating because of the elevated temperatures that are ordinarily required. Further, since sodium is less expensive than calcium its use also serves to reduce the cost of the bath. The lower density of sodium is also operationally advantageous.

In addition, the cleaning of coated metal parts to remove adherent impurities and bath constituents is an important factor in the economics of the process and is of significant concern in the commercial utilization of the process. Water is ordinarily preferred for use for cleaning the coated metal parts because it is an inexpensive and convenient cleaning agent. Where coatings are obtained from a calcium bath alone, upon cleaning with water an adherent layer or film of calcium hydroxide tends to form on the coated article, which makes the cleaning process a slow and difficult one. Where only a molten sodium bath is used, subsequent cleaning with water is extremely rapid and frequently hazardous. However, where the bath composition contains between and 50 percent by weight of sodium, cleaning proceeds at a satisfactory rate, without safety hazards, and without the formation of adherent hydroxide coatings to interfere with the cleaning process.

A particularly preferred calcium-sodium composition is the low-melting eutectic that is formed. Thus the melting point of calcium is 1558 F. (848 C.), whereas the melting point of the calcium (86- weight percent)-sodium (14 weight percent) eutectic is 1310 F. (710 C.). Other bath compositions containing between 5 and 50 percent by weight of sodium may also be used.

Advantageously, using the calcium-sodium bath, particularly the eutectic mixture, suitable chromium diffusion coatings may be obtained at operating temperatures as low as 1400 F. Under similar conditions with sodium alone, no coating is obtained.

There will usually be actual diffusion of the chromium into the base metal, with resulting formation of a solid solution, alloy, or intermetallic compound that is chromium rich at the surface. Diffusion layers with graded boundary layers varying from 0.1 mil up to 60' mils may be obtained, although diffusion coatings typically varying from 0.5 mil to mils are preferred. The diffusion of the coating'material into the base metal accounts for the changed metallurgical properties of the resulting article, for instance improved mechanical and chemical properties. The resulting diffusion layer has a number of distinct advantages over a simple plating layer, primarily due to the fact that the coating is graded in composition. There is no sharp interface, which is beneficial in preventing spalling or breaking of the coating. With a ferrous metal base, a chromium-iron or chromium-nickel-iron alloy is formed. The oxidation and corrosion resistance of such diffusion-coated ferrous metals is very great, while the cost is considerably less than that of stainless steel.

'.While the mechanism of the several steps of the process, such as solution, transport, deposition and diffusion of the chromium into the base metal, as well as the formation and nature of the alloy diffusion layer (e.g., intermetallic compound or solid solution) is not fully understood, the use of the molten calcium-sodium bath is an essential feature of this process. Because of the very high negative free energy of formation of calcium oxide, extremely clean oxide-free surfaces are maintained on the base metal, initially and throughout the coatiirg operation. The calcium-sodium molten bath is less expensive that either calcium, priced at approximately $2/lb., or lithium, priced at approximately $l0/=lb., since sodium is priced at about 16 cents/lb. Where dragout losses occur the use of the molten calcium-sodium bath is particularly economical. The calcium-sodium bath, e.g., the 86-14 eutectic mixture, is protected from oxidation by maintaining an inert environment thereover, i.e., a

non-oxidizing one, such as may be obtained under vacuum or by maintaining an inert gaseous atmosphere pro vided by a noble gas such as helium or argon.

The base metal may be any metal which does not dissolve in the molten bath at an appreciable rate for the time of immersion used. Of particular interest are the ferrous metals in view of their wide usage and nickel and cobalt compositions (i.e., the first triad of Group VIII). Also of interest are the refractory metals for example various Group IV B metals such as titanium, Group V-B metals such as niobium, and Group VI-B metals such as molybdenum and tungsten. Ordinarily there would be no interest in coating chromium per se with a protective coating of chromium. Reference made herein to the various groups of elements refers to the Periodic Chart of the Elements as shown in Handbook of Chemistry, 10th edition, N. A. Lange, editor, McG-raw-Hill Book Company, New York, 1961, pages 56-57.

However, because of the commercial importance of the chrornizing of ferrous base metals, the details of the present invention will be exemplified by reference thereto. The calcium-sodium bath must be maintained in a molten condition. Where the calcium-sodium eutectic is used, the lower limit may be 1350 F. A preferred range for depositing chromium coatings on ferrous base metals is between 1400 and 1800" -F. This upper limit is gen erally dictated by the capabilities of the container materials used in the process since the corrosive activity of the molten transfer agent increases with temperature. Also, since the vapor pressure of the calcium-sodium bath is relatively low at 1800 F., there is little if any loss from an open bath due to evaporation at this temperature and no need to provide for pressurization. Accordingly, a higher upper temperature may be used where desired. The chromium values dissolved in the molten bath will generally constitute from 1 to 20% by weight of chromium based on the calcium-sodium content. The chromium values are preferably dissolved in the molten bath either in the form of chromium metal, which requires pulverizing the chromium into suitable powdered form for ease of solution, or in the form of chromium oxide (012 0 which is generally available in the form of powder and readily dissolves in molten calcium. Where chromium oxide is added, approximately 1.5 to 30% by weight of chromium oxide will be added to give the equivalent amount of chromium metal. Nickel may be additionally added to the bath in the form of metal powder or shot, or in the form of its oxide. Because of the high negative free energy of formation of calcium oxide, any chromium oxide or nickel oxide is almost immediately reduced to chromium or nickel, respectively. The amount of chromi um and nickel values dissolved in the bath will depend to some extent on the surface area of the specimen to be coated.

Diffusion times varying from 30- minutes to 10 hours may be employed in this process, a coating of approximately 0.5 mil being obtained in about 3 hours and a coating of about 1.5 mils being obtained in about 5 hours under typical conditions. It will of course be recognized that the rate of deposition of the diffusion coating is not a linear function but depends upon the relationship and interaction of many factors not fully understood. In actual practice, the rate of diffusion appears to vary logarithmically with time. The time required for the coating of the base metal will further be a function of the temperature of the bath and the concentration of the chromium as well as the coating thickness desired.

Agitation of the molten calcium-sodium bath will be another factor affecting the time required to form a satisfactory diffusion coating of chromium of a given quality and thickness on the ferrous base metal. This agitation may be conveniently accomplished by mechanical agita-- tion of the react-ion chamber and its contents, for ex ample by use of rotating and see-saw type capsules. Al-- ternatively, the capsule or container and its contents may be vibrated, or the contents only may be agitated, or the liquid only may be agitated.

A batch, semi-continuous, or continuous diffusion coating process may be used in the practice of this invention. Conveniently, for all processes any container non-reactive with the molten calcium-sodium may be used to hold the bath. Convenient materials are alloy steel, stainless steel, niobium, and tantalum. In batch processing, specific capsule configurations may also be used to support some of the constituents, to provide a reservoir, or to house unusually shaped specimens. The specimen to be coated may be placed into the bath loose, suspended on wire, encased in a screen envelope or passed through the bath, in the case of semicontinuous and continuous processing.

Various types of furnaces may be used to house the batch-processing capsule, the simplest being a static furnace which is temperature controlled to maintain an isothermal environment for the capsule. In other applications, a tube furnace is used inclined at 30 from the horizontal within which the capsule is rotated about its longitudinal axis at about rpm. Additionally, a rocking motion may be added to the rotating motion. For this type of application, a tube furnace is used in which the capsule is rotated at 15 rpm. while the furnace and capsule are both rocked through i from the horizontal about a mid-point pivot.

An open vessel bath may be used for the batch, semicontinuous, or continuous process. The temperature is maintained by a furnace around the vessel containing the bath. A recirculating argon environment is maintained over the bath solution to prevent any oxidation occurring.

The following examples are offered to illustrate the scope, practice, and advantageous features of this invention in greater detail, and are not to be construed as limitations thereof.

Example 1.Use of calcium-sodium eutectic at 1400 F.

Two mild steel specimens (AISI 1015 and A-ISI 4330M) were run in a capsule in which 5 grams Cr O were dissolved in a eutectic mixture of 86 weight percent calcium-14 weight percent sodium, and run in the furnace with a rolling and rocking mot-ion for 2 hours at 1400 F. A case of about 0.4 mil was formed on the materials. The appearance of the specimens was good, and showed uniformity of coating when immersed in a copper sulfate bath.

In a similar capsule run using sodium alone and 5 Weight percent Or O no apparent reaction occurred.

A replication of the run using 15 grams chromium oxide with the calcium-sodium eutectic instead of 5 grams did not change the results obtained.

A bath mixture of 70% Ca-30% Na with 5 g. Cr O produced a 0.4 mil case equivalent to the one formed when using the eutectic mixture.

A reversal of bath constituents so that 86 weight percent sodium and 14 weight percent calcium was present, and run under the same conditions, resulted in no case being formed.

Example 2.Use of chromium metal Mild steel specimens were coated in capsules run at 1400 F. for 2 hours using the calcium-sodium eutectic as the bath and in which chromium metal was dissolved equivalent to 5 grams Cr O A case of 0.30.4 mil thick was obtained.

Example 3.Increase of sodium content of bath The sodium content of the bath was varied from the eutectic mixture (86 grams Ca, 14 grams Na) to a mixture containing 63 grams Ca, 27 grams Na and finally to a mixture containing 40 grams Ca, 40 grams Na. In all instances 8 grams Cr O were added to the bath. Specimens of mild steel (AISI 1015) were run in capsules with a rolling and rocking motion for 5 hours at 1400" F. A case depth approximately 0.5 mil thick was obtained for all specimens.

It will of course be understood that many variations are possible in the practice of this invention, depending upon the coating thickness desired, the base metal used, the chromium material from Which the dissolved chromium values are obtained, as well as the interrelationship between bath concentration, agitation of bath, bath temperature, and coating time, and these variants are therefore considered to lie within the practice of this invention. Accordingly, the scope of this invention should be determined in accordance with the objects thereof and the appended claims.

I claim:

1. The method of forming a diffusion coating containing chromium on a chrominum-alloyable base metal which comprises placing the base metal in a molten calciumsodium bath containing from 5 to 50 weight percent sodium maintained under an inert environment and containing chromium values dissolved therein equivalent to from 1 to 20 percent by weight of chromium based on the calcium-sodium content, and maintaining said base metal in said bath until a diffusion coating containing chromium is obtained on said base metal.

2. The method of claim 1 wherein said molten bath comprises a molten calcium-sodium eutectic mixture.

3. The method of claim 1 wherein said bath and the formed diffusion coating additionally contain nickel.

4. The method of forming a chromium diffusion coating on a ferrous metal which comprises providing a molten calcium bath containing from 5 to 50 Weight percent sodium under an inert environment, dissolving chromium values therein equivalent to from 1 to 20 percent by weight of chromium based on the calcium-sodium content, and placing said ferrous metal in said molten bath until a diffusion layer of chromium is obtained on said ferrous metal.

5. The method of claim 4 wherein said molten bath is maintained at a temperature between 1400 and 1800" F.

6. The method of claim 4 wherein said molten bath comprises a calcium-sodium eutectic mixture.

7. The method of claim 4 wherein said bath and the formed diffusion coating additionally contain nickel.

References Cited UNITED STATES PATENTS 3,184,292 5/1965 Argyriades et a1. 117114 X 3,184,331 5/1965 Carter 117-114 3,186,865 6/1965 Page 117119 3,261,712 7/1966 Carter 117-114 ALFRED L. LEAVITT, Primary Examiner. J. R. BATTEN, 1a., Assistant Examiner. 

